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| United States Patent |
5,573,991
|
|
Sherif
, et al.
|
November 12, 1996
|
Preparation of metal carbide catalyst supported on carbon
Abstract
A process for forming a supported metal carbide catalyst, for example, a
Group VIB transition metal carbide, such as tungsten carbide, which
process comprises the calcination of a carbon support that has been
impregnated with a metal carbide precursor comprising a water soluble salt
of: (1) a cation comprising nitrogen-hydrogen bonded moieties, such as a
guanidine cation; and (2) an anion, such as a tungstate anion, comprising
metal-oxygen bonded moieties, so that upon calcination the product formed
is the metal carbide and the by-products comprise ammonia and carbon
dioxide.
| Inventors:
|
Sherif; Fawzy G. (Stony Point, NY);
Desikan; Anantha N. (Cortland Manor, NY)
|
| Assignee:
|
Akzo Nobel N.V. (Arnhem, NL)
|
| Appl. No.:
|
246533 |
| Filed:
|
May 20, 1994 |
| Current U.S. Class: |
502/177; 423/439; 423/440; 423/441 |
| Intern'l Class: |
C07C 027/22 |
| Field of Search: |
502/177
423/439,440,441,442,437 R,352
|
References Cited
U.S. Patent Documents
| 4271041 | Jun., 1981 | Boudart et al. | 502/177.
|
| 4325843 | Apr., 1982 | Slaugh et al. | 502/177.
|
| 4522708 | Apr., 1982 | Leclercq et al. | 208/136.
|
| 4851206 | Jul., 1989 | Boudart et al. | 423/440.
|
| 5071813 | Dec., 1991 | Kugler et al. | 502/177.
|
| 5321161 | Jun., 1994 | Vreugdenhil et al. | 564/490.
|
| 5338716 | Aug., 1994 | Triplett et al. | 502/177.
|
| 5451557 | Jun., 1981 | Sherif | 502/177.
|
Other References
Journal of Catalysis 78, 116-125 (1982).
|
Primary Examiner: Pal; Asok
Attorney, Agent or Firm: Fennelly; Richard P., Morris; Louis A.
Parent Case Text
RELATED APPLICATION
This is a continuation-in-part of U.S. Ser. No. 201,475, filed Feb. 24,
1994 U.S. Pat. No. 5,451,557.
Claims
We claim:
1. A process for forming a supported metal carbide catalyst which comprises
the calcination of a carbon support that has been impregnated with a metal
carbide precursor which comprises a water soluble salt of: (1) a cation
comprising nitrogen-hydrogen bonded moieties; and (2) an anion comprising
metal-oxygen bonded moieties, so that upon calcination the product formed
is the metal carbide and the by-products comprise ammonia and carbon
dioxide.
2. A process as claimed in claim 1 wherein the precursor comprises a Group
VIB transition metal.
3. A process as claimed in claim 1 wherein the precursor comprises tungsten
as the metal.
4. A process as claimed in claim 1 wherein the precursor comprises a
tungstate anion.
5. A process for forming a metal carbide catalyst which comprises the
calcination of a carbon support that has been impregnated with a metal
carbide precursor which comprises a water soluble salt of: (1) a guanidine
cation; and (2) an anion comprising the metal and oxygen, so that upon
calcination the product formed is the metal carbide and the by products
comprise ammonia and carbon dioxide.
6. A process as claimed in claim 5 wherein the precursor comprises a Group
VIB transition metal.
7. A process as claimed in claim 5 wherein the precursor comprises tungsten
as the metal.
8. A process as claimed in claim 5 wherein the precursor comprises a
tungstate anion.
Description
BACKGROUND OF THE INVENTION
A variety of disclosures exist in the art in regard to how to form a
supported metal carbide catalyst, including the following:
The impregnation of a support with a water soluble source of the metal
alone, followed by calcination to the metal oxide, with subsequent
exposure of the oxide to carburizing gases, such as methane/hydrogen (See
S. T. Oyama et al., Ind. Eng. Chem. Res., 27, 1639 (1988)) or carbon
monoxide (See P. N. Ross, Jr. et al., J. of Catalysis., 48, 42 (1977)) are
two ways in which such supported catalysts might be formed. Both
carburization reactions necessitate the use of high temperatures on the
order of about 900.degree. C. L. Leclercq et al., in U.S. Pat. No.
4,522,708, discusses several supported carbide systems, including work by
Mitchell and co-workers in supporting molybdenum on active carbon and
other work relating to Group VI metals on alumina (e.g., U.S. Pat. Nos.
4,325,843 and 4,326,992). U.S. Pat. No. 4,325,842 to L. H. Slaugh et al.
describes the preparation of supported molybdenum carbide compositions
which are formed on a variety of supports, including charcoal and
graphite, by impregnating the support with a solution of hexamolybdenum
dodecachloride, followed by drying, and then carburization in a carbiding
atmosphere at elevated temperature.
Pending U.S. Ser. No. 156,670, filed Nov. 23, 1993 teaches that catalytic
metal carbide compositions can be formed by the calcination of a guanidine
compound, derivative or adduct with a transition metal salt containing the
desired metal component of the carbide.
An improvement of the technology described in this pending application is
described in U.S. Pat. No. 5,451,557 in which a water soluble precursor is
calcined to form the desired metal carbide catalyst on a support which is
an oxidic support, preferably coated with a protective ceramic passivation
layer as described and claimed in U.S. Pat. No. 5,338,716.
DESCRIPTION OF THE INVENTION
This invention relates to a novel process for forming a supported metal
carbide catalyst involving the calcination of a carbon support material
which has been impregnated with a water soluble precursor for the metal
carbide. The precursor is made by a one-step chemical reaction between a
transition metal-containing compound and a carbon containing compound
which is low in carbon content as described and claimed in U.S. Pat. No.
5,451,557 which is mentioned hereinbefore. In the broadest embodiment of
the invention, the precursor used in the process is a water soluble salt
of: (1) a cation comprising nitrogen-hydrogen bonded moieties with a high
nitrogen to carbon content; and (2) an anion comprising metal-oxygen
bonded moieties, so that upon calcination the product formed is the metal
carbide and the by-products comprise ammonia and carbon dioxide. The
precursor contains both a metal source, such as the Group VIB transition
metal, tungsten, and a carbon source, such as a guanidine compound.
Guanidine carbonate, which is of the formula (CN.sub.3 H.sub.5).sub.2
H.sub.2 CO.sub.3, contains only 20% carbon and is an example. The
precursor in very soluble in water. Its high solubility allows for the
preparation of carbon supported catalysts in accordance with the present
invention by impregnation. It allows the use of less volume of a solvent,
not exceeding the pore volume of the carbon support that will be
impregnated with the solution. This process, which is known as the
"incipient wetness method", is preferred for the manufacture of such
supported catalysts. The incipient wetness method requires that the volume
of the solution be equivalent to the pore volume of the carbon support.
Upon calcination of the impregnated carbon support, equal dispersion of
the active component on the support will result. High solubility of the
precursor will also allow increasing the metal loading on the carbon
support at will.
In order to synthesize a high surface area, high porosity tungsten carbide
catalyst, supported on a carbon support, it is preferred to have a
precursor of the metal carbide in a form, soluble in water. The precursor
is preferably a single compound and not a mixture of two components. It
does not produce excess carbon upon calcination that would block the
catalytic pore properties of the carbon support material itself. The
composition of the precursor is made from one metal to one to five carbon
atoms, preferable three carbon atoms. The carbon atoms are directly bonded
to nitrogen atoms. The nitrogen atoms may be connected to hydrogen or
other atoms. The carbon-nitrogen entity forms a cation attached chemically
to the metal in the form of an anion with the cation having a high
nitrogen to carbon content, preferably at a nitrogen to carbon atomic
ratio of 3.0 to 1 or higher. The reaction can be described by the
following generalized (unbalanced) equation:
##STR1##
Calcination of this type of precursor, for example, guanidinium tungstate,
at temperatures of from about 500.degree. C. to about 800.degree. C. gives
tungsten carbide (W.sub.2 C) in substantially pure form. If exposed to
air, this material may form a monolayer of W.sub.2 CO. During the
calcination, the precursor components will interact, whereby the organic
source will reduce the metal ion source within the same molecule and form
a metal-carbon bond as a metal carbide, which would be substantially free
of undesirable free carbon as represented by the following generalized
(unbalanced) equation:
##STR2##
This calcination step does not involves carburization. It is a chemical
reduction of the metal ion with the carbon-nitrogen ion of the same
compound or from the ammonia released thereafter. It is believed that
ammonia will reduce the tungstate ion into a lower oxidation state, which
will in situ chemically react with the carbon in the same molecule forming
carbides. The result is a metal carbide containing substantially no excess
carbon, which is well dispersed over the carbon support, giving a high
surface area catalyst. The ratio of the guanidine to the metal was found
to be important for forming a metal carbide suitable for use as a catalyst
for reactions such as isomerization of n-heptane. For example, it will be
shown later that if the ratio of guanidine to the metal is less than
three, other phases such as nitridic or metallic phases will be the main
component. These phases will result in the undesirable cracking of heptane
to lower hydrocarbons. When the ratio is 3:1, the only phase would be
W.sub.2 C. The solution of guanidinium tungstate would also be easily
impregnated into the porous support in one step, then calcined at an
industrially reasonable temperature not exceeding 800.degree. C.
The type of precursor which is to be used in accordance with the present
invention in its broadest embodiment comprises a guanidinium cation and a
transition metal-containing anion with the guanidinium to transition metal
ratio being at least about 3:1, preferably about 3:1. The transition metal
can be a Group VIB transition metal, such as tungsten, and the preferred
anion is a tungstate. A molybdate anion can also be selected.
A composition of matter which is formed by the solid state reaction of
ammonium metatungstate and guanidine carbonate at 100.degree.-200.degree.
C., was found to be: (1) completely soluble in water; (2) decomposable at
228 .degree. C., which is different from decomposition temperature of the
reactants; (3) contains 35-50% tungsten; (4) contains 5-10% carbon; (5)
contains 20-30% nitrogen; (6) has a characteristic X-Ray diffraction
pattern not found before; (7) forms mainly tungsten carbide, W.sub.2 C,
containing substantially no free carbon, when heated at
600.degree.-850.degree. C. under nitrogen, such carbide showing catalytic
activity in chemical hydrotreating reactions known to occur with noble
metals, such as platinum and palladium; and (8) aqueous solutions of
composition are dry impregnable by the incipient wetness method into a
carbon solid support, in accordance with the present invention, prior to
calcination, making it possible to produce such supported metal carbide
catalysts.
The support material that is to be used with the present invention includes
those carbon supports that are known to persons of ordinary skill in the
art in supporting transition metal carbide catalysts, for example, that
have been synthesized by differing methods heretofore.
The following Examples further illustrate the present invention.
EXAMPLE 1
Tungsten carbide supported on activated carbon (Norit.RTM. A activated
carbon) was prepared by impregnating the activated carbon with an aqueous
carbide precursor containing ammonium metatungstate (Sylvania brand 99.9%)
and guanidine carbonate (Aldrich brand 99%). The impregnated samples were
then dried at 150.degree. C. for one hour and then calcined in an inert
atmosphere of N.sub.2 at 600.degree. and 650.degree. C. The samples were
characterized by surface area measurements, x-ray diffraction (XRD),
thermogravimetric analysis (TGA) and selective chemisorption. XRD
indicated the presence of a mixture of amorphous material and very low
crystallinity phases. The active species was identified as an
oxygen-substituted W.sub.2 C. The catalysts had surface areas of about 550
m.sup.2 g.sup.-1.
EXAMPLE 2
This Example demonstrates the reforming activity of the catalysts described
in Example 1. The reforming of n-heptane was performed at 350.degree.14
500.degree. C. and atmospheric pressure. Hydrogen (10 cm.sup.3 min.sup.-1)
was bubbled through a reservoir containing n-heptane at 30.degree. C.
which then flowed through the sample bed (0.5 g). Reactant and product
feed streams were analyzed by gas chromatography. Prior to catalytic
testing, the catalysts were reduced in H.sub.2 at 500.degree. C. for two
hours. The conversions and various selectivities that were obtained are
summarized in the Table below:
______________________________________
Select.
Temp. Time Conversion (%)
(.degree.C.)
(min) (%) C.sub.1 -C.sub.4
Isomers
Aromatics
______________________________________
350 30 2 67 33 --
400 55 14 56 41 3
400 75 13 55 41 4
450 115 49 38 22 40
450 155 44 37 24 39
500 60 94 27 -- 73
500 80 94 26 -- 74
500 100 92 25 -- 75
500 150 90 24 1 75
500 180 89 24 1 66
______________________________________
The data show that at the high temperature of 500.degree. C., about 90% of
n-heptane was converted. The product distribution at this high conversion
was about 75% of the desired aromatic products, mainly toluene, xylenes
and benzene, and about 25% of low hydrocarbons, namely, methane, ethane,
propane, and butane.
EXAMPLE 3
This Example demonstrates the hydrogenation activity of the catalysts. In
this Example, 1-hexene hydrogenation to n-hexane was performed at
135.degree. C. and atmospheric pressure. Hydrogen (10 cm.sup.3 min.sup.-1)
was bubbled through a reservoir containing 1-hexene at 30.degree. C. Prior
to catalytic testing, the catalysts were reduced in H.sub.2 at 500.degree.
C. for two hours. The results are summarized in the Table below:
______________________________________
Temperature Selectivity (%)
(.degree.C.)
Time (min)
Conversion (%)
n-hexane
______________________________________
135 17 100 100
135 50 100 100
______________________________________
The high conversion of 100% and high selectivity of 100% is typical of that
known for noble metal catalysts. Therefore, the instant catalyst could be
an attractive and less expensive alternative to such noble metal
catalysts.
EXAMPLE 4
This Example is similar to Example 1, except that the carbon support was
acid washed activated carbon (Darco.RTM.). XRD indicated the presence of a
mixture of amorphous material and very low crystallinity phases. The
active species was identified as an oxygen-substituted W.sub.2 C. The
catalyst had a surface area of about 924 m.sup.2 g.sup.-1.
EXAMPLE 5
This Example demonstrates the n-heptane reforming activity of the catalyst
described in Example 4. Experimental conditions are similar to that in
Example 2. The conversions and selectivity's are summarized in the Table
below:
______________________________________
Tem- Selectivity
perature
Time Conversion (%)
(.degree.C.)
(min) (%) C.sub.1 -C.sub.4
Isomers Aromatics
______________________________________
350 30 1 -- 80 20
350 50 1 -- 80 20
400 90 4 21 74 5
400 110 4 22 74 4
400 200 3 21 76 5
400 260 3 22 74 4
500 20 72 23 8 69
500 50 59 26 10 64
500 80 60 25 9 66
500 115 54 25 10 65
______________________________________
The data shows that at high conversions of n-heptane (60%), the product
distribution was about 65% of the desired aromatic products, mainly
toluene (79%), benzene (19%) and xylenes (3%), and about 25% of low
hydrocarbons (methane, ethane, propane and butane).
EXAMPLE 6
This Example demonstrates the 1-hexene hydrogenation of the catalyst
described in Example 4. Experimental conditions for the reactivity testing
are identical to those described in Example 3. The conversions and
selectivities are summarized in the Table below.
______________________________________
Temperature Selectivity (%)
(.degree.C.)
Time (min)
Conversion (%)
n-hexane
______________________________________
150 30 90 66
150 60 90 67
______________________________________
The foregoing Examples are presented for illustrative purposes only. The
scope of protection is set forth in the claims which follow.
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